Spain mounted a sweeping anti-terrorism operation on Friday after a suspected Islamist militant drove a van into crowds in Barcelona, killing 13 people, in what police suspect was one of a planned wave of attacks.

Islamic State claimed responsibility for the deadly rampage along the city's most famous avenue on Thursday, which was packed with tourists taking an afternoon stroll. The death toll could rise, with more than 100 injured, authorities said.

As security forces hunted for the van's driver, who was seen escaping on foot, police said they had killed five attackers on Thursday night in Cambrils, a town south of Barcelona, to thwart a separate attack using explosive belts.

Six civilians and a police officer were injured in Cambrils when the attackers ran them over in a car, before police shot them dead and carried out controlled explosions. Police said the Cambrils incident was linked to the van attack in Barcelona.

Before the van ploughed into the tree-lined walkway of Las Ramblas, one person was killed in an explosion in a house in a separate town southwest of Barcelona, police said. Residents there were preparing explosives, a police source added.

Police said they had arrested a Moroccan and a man from Spain's north African enclave of Melilla, though neither was the van driver. A third man was arrested in the town of Ripoll on Friday in connection with the attack.

It was still not clear how many people had been involved in the van attack and other incidents on Thursday.

Witnesses to the van attack said the white vehicle had zigzagged at high speed down Las Ramblas, ramming pedestrians and cyclists, sending some hurtling through the air and leaving bodies strewn in its wake.

The injured and dead came from 24 different countries, the Catalan government said on Friday in a statement, ranging from France and Germany to Pakistan and the Philippines. Spanish media said several children were killed.

(To view a graphic on the Barcelona crash, click http://tmsnrt.rs/2fOJ9Sm)

ISLAMIC STATE CLAIM

Islamic State's Amaq news agency said: "The perpetrators of the Barcelona attack are soldiers of the Islamic State and carried out the operation in response to calls for targeting coalition states" - a reference to a U.S.-led coalition against the Sunni militant group.

Spain has several hundred soldiers in Iraq providing training to local forces in the fight against Islamic State, but they are not involved in ground operations.

The Islamic State claim could not immediately be verified.

If the involvement of Islamist militants is confirmed, it would be the latest in a string of attacks in the past 13 months in which they have used vehicles to bring carnage to the streets of European cities.

That modus operandi - crude, deadly and very hard to prevent - has killed well over 100 people in Nice, Berlin, London and Stockholm.

British tourist Keith Welling, who arrived in Barcelona on Wednesday with his wife and 9-year-old daughter, said they saw the van drive past them down the avenue and took refuge in a restaurant when panic broke out and the crowd started running.

"People were shouting and we heard a bang and someone cried that it was a gunshot ... Me and my family ran into the restaurant along with around 40 other people.

"At first people were going crazy in there, lots of people crying, including a little girl around three years old."

It was the deadliest attack in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.

Spanish Prime Minister Mariano Rajoy announced three days of official mourning for what he called a "jihadist attack."

The Spanish royal household said on Twitter: “They are murderers, nothing more than criminals who are not going to terrorise us. All of Spain is Barcelona.”

U.S. President Donald Trump said: "The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help."

He added: "Be tough & strong, we love you!"

BODIES ON THE GROUND

Police said the two men detained on Thursday had been arrested in two towns, Ripoll and Alcanar, both in the region of Catalonia, of which Barcelona is the capital.

The explosion was also in the town of Alcanar, in the early hours of Thursday. One person died and another was injured in that incident, police said.

Police said they also shot dead on Thursday a man who had driven a car into a police checkpoint in Barcelona, though they had no evidence he was connected with the van attack.

Mobile phone footage showed several bodies strewn along the Ramblas, some motionless. Paramedics and bystanders bent over them, treating them and trying to comfort those still conscious.

Around them, the boulevard was deserted, strewn with rubbish and abandoned objects including hats, flip-flops and a pram.

German television channel ZDF reported that three Germans were among those killed, and Belgium's foreign minister said a Belgian was among the dead.

France said 26 of its citizens were hurt, and 11 of them were in a serious condition. Australia said at least four of its nationals were injured, and Italy three.

Regional head Carles Puigdemont said people had been flocking to hospitals in Barcelona to give blood.

Susana Elvira Carolina, 33, who works at a shop on Las Ramblas, had just entered her building when the van struck.

"We had a window and you could see the bodies lying from there, you could see how people were run over ... We were shutting down the blinds but people kept coming in and we had to keep it open so they could enter the shop."

TOURIST DRAW

The atmosphere on Las Ramblas was subdued on Friday morning, as media crews mingled with tourists. Police tape cordoned off some areas of the avenue.

The incident took place at the height of the tourist season in Barcelona, which is one of Europe's top travel destinations with at least 11 million visitors a year.

French President Emmanuel Macron, whose nation has suffered some of Europe's deadliest militant attacks in recent years, tweeted: "All my thoughts and France's solidarity to the victims of the tragic attack in Barcelona."

Other governments also expressed condemnation and sympathy. Polish Interior Minister Mariusz Blaszczak said the attack showed the European Union's system of migrant relocation was wrong.

"It is dangerous. Europe should wake up," he said. "We are dealing here with a clash of civilisations."

Authorities in Vic, a small town outside Barcelona, said a van had been found there in connection with the attack. Spanish media had earlier reported that a second van had been hired as a getaway vehicle.

Barcelona is the capital of the wealthy northeastern region of Catalonia, which plans to hold a popular vote on Oct. 1 on whether it should secede from Spain. The central government says the vote cannot go ahead because it is unconstitutional.

Before Thursday's attack, government data showed that police had arrested 11 suspected jihadists in the Barcelona area so far this year, more than anywhere else in Spain.